Tabula Rasa (literally, "erased slate," although "blank slate/page" is more often heard) is the concept that all minds start out completely empty of knowledge and information, save the animal instincts. The idea is simply that human beings learn from experience and information, instead of starting with a basic amount of knowledge and building on that. It is very similar to the common religious concept of "infant purity," or the idea that a child has no sin because they are pure from birth, and only living and activity gives them opportunity to sin (Christianity notably averts this concept).
This is part of the philosophical area known as Epistemology, or study of knowledge, which is a constant field of argument and debate depending on which philosophical field one follows. The most inherent question asked here is, "How do we know what we know?" According to tabula rasa, we know only because we have learned or experienced, and there is no such thing as "innate" knowledge or skill. John Locke was a proponent of this theory.
Tabula rasa is used to understand how a person can be raised in an environment and thus become a creature of that environment; babies from the same family can be raised in different places and grow to be vastly different people, even if they are born as twins. It is also used to understand amnesia and brain trauma, which can wipe a person's brain of experience or knowledge and leave them susceptible to new information. It is philosophically related to Empiricism, Rationalism, Nature vs. Nurture, and Innatism.